[FEATURE] Support multiple simultaneous Notion workspace connections in one session

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

The Notion MCP connector authenticates to a single Notion workspace at a time. I work across two separate Notion workspaces under the same account (two businesses I run in parallel, kept in separate workspaces on purpose). To switch between them inside a Claude Code session, I have to manually disconnect and reconnect the Notion integration every time — there's no way to hold both connections live at once.

### Proposed Solution

Allow a single Claude Code session to access multiple Notion workspaces at the same time — for example by letting the Notion connector be added twice, each instance scoped to a different workspace, or by supporting a per-request workspace selector so switching doesn't require a full disconnect/reconnect cycle.

### Alternative Solutions

A lighter alternative: keep a single connector, but allow specifying which workspace to target per session without going through Notion's full re-authorization flow each time — e.g. a quick workspace-switch command.

### Priority

High - Significant impact on productivity

### Feature Category

MCP server integration

### Use Case Example

I run two independent businesses (a delivery consulting practice, a separate strategy consulting practice, online product), each with their own dedicated Notion workspace under the same account. I work on both in parallel throughout the day. Every time I need to switch context, I have to disconnect the Notion integration from workspace A and reconnect it to workspace B, which breaks my flow and wastes time.

### Additional Context

Notion account: Email.com — two workspaces used daily and in parallel from the same Claude Code account.

The issue identifies the Notion MCP connector and Claude Code session as the scope, but names no files, tests, or entry points. First map how MCP connections are registered and authenticated, then determine how workspace identity is stored and selected; done means two workspaces remain usable concurrently without disconnecting or reauthorizing.

索引モデルが issue の本文から書いたものです。